OBJECTIVE: This study aimed to establish cephalometric reference values for mandibular symphysis in adults. Dentoalveolar, skeletal and soft tissue variables were measured considering the influence of gender and facial type. METHODS: The sample consisted of sixty cephalometric radiographs of white Brazilian adult patients, with a mean age of 27 years and 6 months, who had not undergone orthodontic treatment and who presented well-balanced faces and normal occlusion. The sample was standardized according to gender (30 males and 30 females) and facial type (20 were dolichofacial, 20 mesofacial and 20 brachyfacial). RESULTS: The results showed that male and female symphyses are similar, except for symphyseal height, which was greater in males. In terms of facial type, the dolichofacial group presented narrower symphysis in dentoalveolar and basal areas, with a more accentuated lingual dentoalveolar inclination.CONCLUSION: The brachyfacial group showed broader symphysis in the dentoalveolar and basal areas and a greater buccal dentoalveolar inclination. The projection of the chin was 6.67 mm below the subnasal vertical line and there was no significant difference between the genders or facial types.

Keywords: Mandibular symphysis. Gender. Facial type. Facial balance.

INTRODUCTION

Mandibular symphysis is an anatomical structure of the mandible in which the lower incisors are found including the anterior portion of the chin. Mandibular symphysis contributes to the composition and balance of facial harmony2,15,25 and must be considered when deciding on orthodontic treatment in borderline cases.12,20,30

Mandibular symphysis is morphologically divided into two regions, the dentoalveolar and basal symphyses.22 The dentoalveolar symphysis includes the alveolar process and lower incisors. The long axis of the lower incisors cephalometrically matches the long axis of the alveolar process22 and its inclination is influenced by facial type.16,29 This classical concept dates from the Tweed era and defines the lingual inclination of the alveolar long axis (IMPA) in subjects with a high mandibular plane (FMA), while in subjects with low mandibular planes, the long axis is more buccally tipped.29 According to this view, the positioning error of the lower incisors could compromise the stability of orthodontic results and facial esthetics.29

Alveolar bone thickness varies according to location and facial type.12 Generally, there is a greater bone thickness at the apex then in the cervical region, and towards the lingual surface when compared to the labial surface.12 This explains the higher prevalence of bone dehiscence and fenestration on the buccal side, and gives rise to periodontal concern about the anterior orthodontic movement of the lower incisors.8

However, studies related to buccal projection3,4,9,10,19,28,30 of lower incisors present conflicting results, probably due to methodological differences and limitations, and the multifactorial etiology of periodontal recession.31 However, thin buccal bone coverage of the root10,12,28 associated to excessive buccal movement31 and insufficient thickness of the marginal gingiva have been shown19,31 to be significant variables in the development of non-inflammatory gingival recession.

In terms of cortical bone, the lingual side is thicker than the buccal, and due to the inclination of the lower incisors, there is a closer approximation of the root apex to the lingual cortical. This apex relationship is particularly evidenced in subjects with vertical growth tendency12 and Class III malocclusion.12,22 since the alveolar bone is very narrow in this region. Bone in the referred apical region is assumed as non-remodelable anatomical limit and restricts the orthodontic retraction movement, because it can perforate the lingual cortical.12,20,24

The basal symphysis is part of the main body of the mandibular symphysis with more apical location, setting the hard menton outline. The menton is considered to be a recent phylogenetic acquisition (just over 10,000 years ago), exclusive to Homo sapiens. The morphological variation of the menton has a strong genetic basis and its occurrence may have emerged casually14 and, did not add any biomechanical advantages for mastication.

The long axis of the basal symphysis differs cephalometrically from that of the alveolar symphysis.22 Tooth movement of the lower incisors cannot influence the shape or position of the basal symphysis. The relationship between the height and width of the mandibular symphysis is one of Björk's five criteria for establishing the mandibular rotation pattern during growth.1,5,6,27 For long and narrow symphyses, the tendency of mandibular rotation during growth is predominantly vertical; when short and wide, it is predominantly horizontal.5 In the vertical pattern, a mandibular symphysis with a long axis and greater lingual inclination has also been observed.12,16

The morphology of the mandibular symphysis is also influenced by the sagittal growth pattern.12,16,22 In Class III malocclusion, a higher,22 narrower12 symphysis with greater anterior projection16 and evident lingual inclination of the long axis has been identified.16,22

In addition, the height and projection of the basal symphysis influence the position of the adjacent soft tissue and are significant in terms of aesthetic and facial harmony.2,15,25 Menton deformities can be treated satisfactorily using basilar genioplasty. For this procedure, it is necessary to establish normative values for height and anterior projection, that are both influenced by ethnicity and sexual dimorphism. These values are usually higher in males.2

Despite its relevance, few studies have focused on mandibular symphysis17,26 and its standard cephalometric values. Some studies lack for uniformity in the sample regarding ethnicity, facial pattern and malocclusion. Hence, the objective of this study was to describe the morphology of the mandibular symphysis in a sample of Brazilian adults with well-balanced faces and normal occlusion, individualized in terms of gender and facial type variables.

SUBJECTS AND METHODS

The research project was submitted to the Research Ethics Committee of Universidade Federal de Uberlândia and approved under the protocol number 247/07.

Sample selection

The total sample, composed of 60 subjects with well-balanced faces, equally divided between the genders, was prospectively selected from students of the Federal University of Goiás Dental School and complemented with subjects retrospectively selected from patients with minimum morphological occlusion deviations from the researchers' private clinics. The mean age of participants was 27 years and 6 months. The sample was also evenly distributed between the possible vertical variations in terms of facial type (dolichofacial, mesofacial and brachyfacial) (Table 1).

The following inclusion criteria had to be fulfilled by all participants: 1) be Brazilian; 2) Caucasian; 3) males over 18 and females over 16; 4) ANB between 0º and 4º; 5)well-balanced face; 6) apparent facial symmetry (clinically determined); 7) normal occlusion with Class I canine and molar relationship, overjet and overbite up to 3 mm and crowding up to 4 mm; 8) presence of all teeth, except third molars; 9) no serious medical condition; 10) no history of facial or dental trauma; 11) no previous orthodontic or prosthetic treatment, facial plastic surgery or orthognathic surgery.

In this study, all the subjects showed a well-balanced face according to Capelloza's Pattern I description.7,23 There were no skeletal discrepancies in sagittal or vertical directions, and the profile was orthognathic, in other words, with gentle facial convexity, lips sealed when resting, the proportion of the facial thirds and the upper lip height were equal to half the height of the lower lip. In order to define the facial type, concordance between the subjective facial analysis and the angle of the mandibular plane (SN.GoGn) were used as criteria. Subjects were classified as mesofacial when SN.GoGn was between 30º and 34º, brachyfacial when less than 30º and dolichofacial when greater than 34º. For profile evaluation, the menton-neck line (length and angle) was used. Subjects were characterized as brachyfacial when the line was elongated and the angle more open. For mesofacial subjects, the line was proportional and the angle close to 90º. For dolichofacial subjects, the line was shortened and the angle reduced. For the frontal evaluation, the referential used was the width between gonion landmarks. This reference was comparatively larger for the brachyfacial type, balanced for the mesofacial type and narrow for the dolichofacial type. Cases in which the facial analysis was not compatible with the SN.GoGn angle were excluded from the sample (Fig 1).

Cephalometric method

After the radiographs were taken, the cephalogram was performed by a single calibrated examiner. Ultraphan paper, a 0.5 mm propelling pencil, soft white eraser, ruler, protractor, square (Desetec) and lightbox were used. The tracings were performed using predefined points, lines and planes in a dark room using black cardboard to protect the edges of the radiographic film. The values obtained were rounded off to 0.5 or the nearest whole number when decimal values were found. Radiographs were excluded when it was impossible to identify anatomical design.

» IIiAIiMe: Distance from the projection of the long axis of the lower incisors on the mandibular plane to the Me point.

» BBD: Buccal bone distance, comprising the thickness of the buccal alveolar bone at the apex of the lower incisors, measured from the AIi point to the external buccal cortical point, using the path of the IIiAIiperp line.

» LBD: Lingual bone distance, comprising the thickness of the lingual alveolar bone at the apex of the lower incisors, measured form the AIi point to the external lingual cortical point, using the path of the IIiAIiperp line.

» PogPog'': Distance between the pogonian and the lingual pogonian points representing the thickness of the basal symphysis, suggested by Nojima et al.22

» IIiMe: Height of the long axis of the mandibular symphysis.

» Pog'Sn (perpOrPo): Distance from the menton soft tissue to the subnasal line perpendicular to the Frankfurt plane.

Systematic error

In order to evaluate the systematic error, 20 randomly selected radiographs used in this study, were remeasured after 30 days. To determine intra-examiner error, the paired t test was applied. Random error was calculated using Dahlberg's test13 when error values greater than 1.5º or 1.0 mm were found. As noted in Table 2, systematic error was statistically significant for SN.GoGn and SfMe.OrPo, but with a slight average difference (0.67º and 0.62º, respectively), irrelevant from the clinical point of view. The results revealed a random error less than 1.5º and 1.0 mm, indicating the reliability of the data.

Statistical Analysis

Data normality of distribution was verified by the Kolmogorov-Smirnov test. A comparison of cephalometric measurements according to gender and facial type was performed using Student's t test for independent samples and analysis of variance (ANOVA), respectively. When the ANOVA indicated a statistically significant difference, the Tukey test for multiple comparisons was applied. For the statistical treatment of data, the SPSS for Windows (version 16.0) was used, considering a significance level of 5% (a = 0.05).

RESULTS

Composition and characteristics of the sample

The sample consisted of subjects ranging from 18 to 38 years for males and 16 to 35 years for females. All subjects presented well-balanced faces, confirmed by subjective facial analysis and cephalometric measurements. The average ANB angle was 2.16±1.63º, indicating harmony in the sagittal position of both maxilla and mandible, and the average SN.GoGn was 32.11±5.46º), which confirmed facial balance in the vertical position. Classification in terms of facial type was clearly established by SN.GoGn cutoff values (Fig 4).

In this study, the buccolingual inclination of the lower incisors represented the long axis of alveolar symphysis. The cephalometric measurements which contributed to this evaluation were IMPA, FMIA, IIiAIi.POM and IIiAIiMe. In general, the lower incisors were implanted perpendicular to the mandibular base (IMPA = 92.78º), buccally in relation to the Frankfurt horizontal plane (FMIA = 61.13º) and lower occlusal plane (IIiAIi.MOP= 63.10º) and the projection of the long axis of these teeth is about 9.51 mm after the Me point (Table 3).

The amount of buccal and lingual bone at the apex of the lower incisor was measured by BBD and LBD widths, respectively. In this sample, the amount of buccal bone (BBD = 5.12 mm) was thicker than the amount found for lingual bone (LBD= 3.55 mm) (Table 3). The long axis of the basal and alveolar symphyses was not aligned. The basal symphysis was inclined 22º lingually in terms of the dentoalveolar symphysis in relation to both the mandibular and Frankfurt planes (SfMe.GoMe = 70.33±5,44º and SfMe.OrPo = 83.13±6.50º). The width of the basal symphysis baseline was 15.61 mm (PogPog''), considered almost twice (BBD LBD = 8.67 mm) that of the dentoalveolar symphysis at the apex of the lower incisors. Symphysis height (IIiMe) was 44.78±3.79 mm and in terms of soft tissue, the projection of the Pog' remained about 6.7 mm below the vertical subnasal line [Pog'-Sn(perp OrPo)] (Table 3).

Gender

Regarding gender, the results showed no statistically significant difference for most cephalometric measurements. Hence, as a general rule, both male and female mandibular symphyses have a similar morphology, except for a slight inclination of the basal symphysis (SfMe.PoOr) and height (IIiMe).

The basal symphysis inclination in relation to the Frankfurt plane (SfMe.PoOr), was 84.97º for males and 81.28º for females, and this difference was statistically significant at 5% level. However, caution should be exercised when evaluating this finding, because the systematic error was significant for this measurement (Table 4). Mean values for mandibular symphysis height (IIiMe) were 46.97 mm and 42.58 mm, respectively, in both males and females. On average, male mandibular symphysis was 10% higher than female symphysis, and this finding was statistically significant (p < 0.00). Therefore, the height of the mandibular symphysis was considered a distinguishing criterion between the genders (Table 4).

Facial type

Facial type had no correlation with the FMIA, SfMe.GoMe or Pog'Sn (perpOrPo) measurements. IMPA and Pog'Pog'' measurements were similar for brachyfacial and mesofacial types and LBD measurements were similar for mesofacial and dolichofacial types SfMe.OrPo, BBD, IIiAIiMe, IIiMe and IIi.MOP were statistically different for the extreme facial types (dolichofacial and brachyfacial) but similar for the mesofacial type (Table 4).

DISCUSSION

This study described the cephalometric characteristics of the mandibular symphysis of a sample consisted of 60 Brazilian Caucasian adults residents of the central region of the country, with an average age of 27 years and 6 months. Subjects presented well-balanced faces and normal occlusion. The measurements analyzed included dentoalveolar, skeletal and soft tissue structures of the mandibular symphysis and the main objective was to evaluate the influence of gender and facial type on the morphology of the symphysis. In this study, the distinction between facial types was made using concordance between facial analysis and the SN.GoGn value. The cutoff value to characterize the mesofacial type was performed with a slight variation (2.0º) from the normative value (32º). Hence, when the facial features were compatible with a SN.GoGn less than 30º, the type was considered well-balanced brachyfacial and dolichofacial when over 34º. From this sample, it can be seen that reading the SN.GoGn angle is quite adequate for evaluation of facial type, just as Tweed suggested in relation to the FMA angle.29 The data obtained in this study confirmed certain characteristics of the mandibular symphysis already described in the literature, but it also unprecedentedly showed the influence of certain measurements when drawing up individualized therapeutic targets for Brazilians.

Gender

The similarities between male and female mandibular symphyses are evident, except in the case of height. The results in general showed significant morphological similarity between the dentoalveolar and basal symphyses, both in thickness and inclination. The absence of sexual dimorphism for the IMPA angle has also been confirmed by other studies17,26 involving normal occlusion.

The expectation of finding a male symphysis statistically more prominent than the female was not confirmed in this study, same findings were previously reported by Scavone et al25 and Arnett et al.2 The results confirmed that both the width of the basal symphysis and its anterior projection are similar between the genders. The perception of a more projected mandibular symphysis in males may be explained by a greater vertical tendency and especially by its greater height. On average, the height of the mandibular symphysis in males was 47 mm and 42.5 mm in females. This difference was statistically significant (p = 0.0) and can thus be considered a differentiating factor between the genders.

Facial type

In this study, the sample was based on subjects with skeletally well-balanced faces, but with variations in their mandibular plane angles. In addition to a subjective facial analysis, the subjects were categorized into three distinct facial types: dolichofacial, mesofacial and brachyfacial. One of the main objectives of this study was to identify possible variations in the morphology of mandibular symphysis from the premise of a variation in the facial morphology not involving the extremes.

Dolichofacial types presented features well described in the literature,5,6,12,27 which include narrower and higher alveolar and basal symphyses with greater lingual inclination of the lower incisors. For this reason, the projection of the long axis of the alveolar symphysis was closer to the Me point (IIiAIiMe) in the dolichofacial types. These characteristics are typical morphological signs of subjects who are hyperdivergent or also called long faced. This study showed the tendency in the mandibular symphysis morphology in well-balanced dolichofacial type subjects and which probably becomes more accentuated as the vertical gap increases. The average thickness of the alveolar symphysis in the region of the apex of the lower incisors found by Handelman,12 in 1996, in patients with a high mandibular plane was 5.5 mm. This result was lower than the findings of this study for dolichofacial type people with a well-balanced facial pattern (7.32 mm). However, there were methodological differences between the studies, such as the inclusion of patients with malocclusion, extreme vertical growth patterns and the different criteria for measuring the alveolar symphysis.

After adding the mean values of buccal and lingual thickness (BBD + LBD), the dolichofacial type group showed an average of 7.32 mm, while the average for the mesofacial and brachyfacial type groups was 8.72 mm and 9.94 mm, respectively. These values denote that the alveolar symphysis in the apical region of the lower incisors is on average 20% narrower in dolichofacial types.

For brachyfacial well-balanced faces, the most striking morphological feature was the greater thickness of the bone near the apex of the lower incisors, especially at the lingual region (LBD). In general, the findings of this study are in accordance with the literature in terms of a wider and shorter symphysis, with a greater buccal inclination of the dentoalveolar and basal symphyses for brachyfacial types.

The cephalometric IMPA measurement was influenced by facial type. The mean values were 88.27º, 93.42º and 96.65º, respectively, for the dolichofacial, mesofacial and brachyfacial types. Tweed's concept,7,29 is summarized as inclining the incisors and the alveolar portion in the buccal direction as the tendency to grow becomes more horizontal.

In contrast, the FMIA measurement, which evaluates lower incisor inclination in relation to the Frankfurt plane, was less variable with the oscillation of the mandibular plane. According to the results, this angle ranged between 60º and 62º for most patients (Fig 4).

Clinical implications

For the surgical orthognathic planning in cases of menton deformities, a comparison with normative values is needed. Thus, the extent of the surgical movement depends on the pre-surgical measurement of the height and anterior symphysis projection of the face. The height of the mandibular symphysis recommended for male and female Caucasian North Americans is 44 mm and 40 mm, respectively.2 This study found higher mandibular symphyses, 47 mm and 42.5 mm, respectively. In other words, a 10% greater proportion for males was maintained, just the absolute value increased.

The expression of a higher mandibular symphysis and a lesser anterior projection in white Caucasian Brazilians contrasts when compared to North Americans. An average position of 6.67 mm below the subnasal line perpendicular to the Frankfurt plane was found, and it is worth noting that no significant difference was found between the genders. In North American Caucasians2 the value found was 3.5±1.8 mm for males and 2.6±1.9 mm for females, with a differential methodology in the use of the natural head position. However, the lesser projection of the menton in white Caucasian Brazilians has also been confirmed by other studies15,25 (Fig 5).

Because of this difference, the use of normative value guideline of samples from North American Caucasians has been questioned for therapeutic application in white Brazilians.25 This statement can be partly explained by the difference in ethnic origin, as white Brazilian are descendents of people from Mediterranean countries, such as Portugal, Italy and Spain, whereas North American Caucasians are mainly of English, Polish, Dutch, Scottish and French origin. Ethnic and individual diversity in human facial contours in Caucasians from different countries means that normative values25 cannot be applied universally. Another reason to justify this difference is the criterion used for sample selection. Arnett et al2 formed a sample with photographic models, unlike this study and others15,25 whose basis for selection was well-balanced faces, not always associated with beauty. Hence, it is essential to individualize orthodontic planning according to the population group being analyzed.

The thickness of the dentoalveolar symphysis is another feature of clinical relevance and its evaluation can establish the extent of safe orthodontic movement of the lower incisors, such as projection and retraction.24,28 The possibility or lack of possibility of this orthodontic movement helps in making decisions for borderline cases undergoing orthodontic treatment with or without tooth extraction or in the treatment of skeletal sagittal discrepancies with compensation or with orthognathic surgery.12 Buccal and lingual corticals at the level of the incisor apex may represent the lower anatomic limits for orthodontic movement, since there is no bone apposition12,20,28. When tooth movement exceeds the limits imposed by the alveolar symphysis morphology, there could be a risk of instability or iatrogenisis.12,20,30 Hence, severe skeletal discrepancies in narrow alveolar symphyses limit orthodontic compensation and require orthognathic surgery. This concern about mandibular symphysis thickness is particularly acute in dolichofacial types. With the lesser alveolar thickness, subjects with vertical growth are naturally more limited in terms of sagittal orthodontic movement. An example of this clinical difficulty is the planning of this orthopedic treatment in cases of Class II malocclusion with mandibular deficiency and accentuated vertical growth. Mandibular growth with clockwise rotation complicates orthopedic mandibular correction and requires a compensatory projection of the lower incisors in a narrow symphysis. The periodontal prognosis will depend on the quality of local hygiene and mainly on marginal gingival thickness.3,19,31

Orthodontists have traditionally evaluated lower incisor positioning using angular and linear cephalometric measurements. It is important that a morphological analysis of the dentoalveolar symphysis be added to this simplistic geometric analysis. For this reason, computed tomography to evaluate buccal-lingual bone volume and density in the alveolar region of the symphysis prior to orthodontic treatment has become increasingly common.11,18,19,21,24

Considering these facts and recognizing the undeniable importance of the mandibular symphysis for orthodontic treatment, this study has emphasized the need for individualization. It can be concluded that even for well-balanced facial patterns, some morphological variations are influenced by gender and facial type.

CONCLUSIONS

Based on these results and in accordance with the methodology used, it was concluded that:

» Mandibular symphysis height was a differentiator between the genders and was, on average, 10% higher in males.

» The degree of divergence of the mandibular plane tended to influence the inclination of the dentoalveolar symphysis but not that of the basal symphysis.

» Well-balanced dolichofacial types have a narrower mandibular symphysis in the alveolar and basal portions and a greater dentoalveolar lingual inclination.

» Well-balanced brachyfacial types have a thicker mandibular symphysis in the alveolar and basal portions and a greater dentoalveolar buccal inclination.

» The soft tissue projection of the chin was on average 6.66 mm below the subnasal vertical line and there was no distinction between the genders and facial types.

» The authors report no commercial, proprietary or financial interest in the products or companies described in this article. » Patients displayed in this article previously approved the use of their facial and intraoral photographs.